1. Field of the Invention
The present invention relates to an incremental encoder used for audio acoustic devices, measurement instruments and the like. In particular, the present invention relates to a miniaturized incremental encoder obtained by improving the shape of a common terminal.
2. Description of the Related Art
An incremental encoder (hereinafter, referred to as an encoder) is attached to a rotor of a rotating machine such as a motor and a generator, and generates a pulse signal in accordance with the rotation state. The pulse signal is used for detecting the rotation speed of the rotating machine and regulating its rotation.
The structure of a conventional encoder and an example of its output-signal waveform will be described with reference to FIGS. 5 and 6.
The encoder shown in FIG. 5 comprises a rotary contact plate 50, a signal-generating terminal 54 and a common terminal 55.
The rotary contact plate 50 includes a signal pulse-generating region 51 and a common contact region 52 in which the signal pulse-generating region 51 comprises slits 51a and spoke-shaped conductive regions 51b alternately formed therein. The spoke-shaped conductive regions 51b space the respective slits 51a from each other. The number of the slits 51a and the spoke-shaped conductive regions 51b are designed in accordance with the number of pulses which should be generated for each rotation. Each slit 51a is filled with a resin, making the rotary contact plate 50 a plane. The rotary contact plate 50 is fixed to a rotor 53 having a rotary operation axis 53 by an insert molding method or the like.
The signal-generating terminal 54 which comprises a sliding contact 54a for generating an A phase signal and a sliding contact 54b for generating a B phase signal is positioned so as to come into contact with the signal pulse-generating region 51 of the rotary contact plate 50. Further, the sliding contacts 54a and 54b are positioned so as to come in contact with the rotary contact plate 50 with a positional shift angle x corresponding to a phase difference to exist between the A phase signal and the B phase signal.
On the other hand, the common terminal 55 is positioned so as to be always in contact with the common contact region 52 of the rotary contact plate 50, whereby a constant reference potential is provided.
When the rotating machine rotates, the rotary contact plate 50 also rotates through the rotor 53 about the rotary operation axis 53'.
The sliding contacts 54a and 54b are successively connected to the common terminal 55 through the rotary connect plate 50, whereby an output waveform shown in FIG. 6 is obtained. In this case, for example, if the widths of the spoke-shaped conductive region 51b and the slit 51a are 4.degree. and 8.degree. respectively, an A phase-signal waveform 61 and a B phase-signal waveform 62 are plus waveforms having an ON width of 4.degree. and an OFF width of 8.degree. respectively.
In FIG. 6, the phase difference between the A phase-signal waveform 61 and the B phase-signal waveform 62 corresponds to the positional shift angle x between the sliding contact 54a for generating the A phase signal and the sliding contact 54b for generating the B phase signal. Accordingly, in the exemplary encoder shown in FIG. 5, since the positional shift angle x between the sliding contacts 54a and 54b is 2.degree., the phase difference between the A phase-signal waveform 61 and the B phase-signal waveform 62 is also 2.degree..
However, in the above-mentioned conventional encoder, the rotary contact plate 50 must be equipped with the common contact region 52. Thus, the miniaturization of the conventional encoder is limited.
In order to solve this problem and provide a miniaturized encoder in which a plurality of signal pulses having phase differences can be output, Japanese Laid-Open Utility Model Publication No. 1-64015 discloses an encoder requiring no rotary contact plate.
The encoder disclosed in this prior art includes, in place of the rotary contact plate, a signal pulse-generating band and a common contact band provided in the side portions of the rotor 53. The signal pulse-generating band and the common contact band are around the rotor. The signal pulse-generating band is coated with a conductive material and an insulating material alternately and the common truck band is coated with a conductive material.
Further, the encoder includes a plurality of signal-detecting brushes and a common terminal brush, the former being in contact with the signal pulse-generating band and the latter being in contact with the common contact band.
When the rotating machine rotates and then in turn the encoder rotates about the rotary operation axis, each signal-detecting brush alternately touches the conductive material and the insulating material. Since the common terminal brush is always in contact with the conductive material and provides a constant reference potential, a pulse-voltage waveform having a period corresponding to the number of rotation of the rotation axis can be obtained by the combination of the output voltage of the signal-detecting brushes and the common terminal brush. The adjustment of relative positions of the plurality of signal-detecting brushes makes it possible to output a plurality of pulse signal waveforms having the desired phase differences.
Since the encoder disclosed in the prior art requires no rotary contact plate, it is possible to miniaturize the encoder in a radius direction. However, the signal pulse-generating band and the common contact band should be formed on the rotary operation axis, making it difficult to miniaturize the encoder in an axis direction. Oppositely, the encoder might become longer in the axis direction, compared with the conventional encoder.